1. Field of the Invention
The invention relates to an automatic variable-speed transmission for setting up a plurality of gear steps, in particular for motor vehicles.
2. Description of the Related Art
Conventional manual transmissions are designed, as is known, as countershaft transmissions in the spur-wheel form of construction. These transmissions offer high efficiency on account of the form-locking effect of the meshing action. To change the gears, however, the transmission must, as a rule, be set so as to be load-free, with the result that an interruption in traction occurs. This is scarcely noticeable in subjective terms during manual shifting.
For a few years, however, there has also been a tendency towards what are known as automated shift transmissions. In these, gear changes take place by means of actuators. As a rule, this type of transmission is coupled to an actuator-operated starting and separating clutch of conventional form of construction. This transmission admittedly offers even higher efficiency in a mapping-controlled automatic mode. However, in the automatic mode, the interruptions in traction which occur become noticeable in a more unpleasant way in subjective terms. A transmission of this type is known, for example, from DE 196 15 267 A1.
Furthermore, to improve comfort, a transmission was proposed (DE 199 08 602 A1) in which the synchronizing units of the countershaft transmission are designed to have an at least part-load capability. It is thereby possible, in the event of a gear change, to take over the prevailing engine torque on the synchronizing unit of the target gear, without the input-side separating clutch being opened completely. The traction collapse during a shift operation is consequently appreciably mitigated.
Furthermore, in the field of automatic transmissions for motor vehicles, there are what are known as “classic” automatic transmissions. These have a hydrodynamic torque converter and, as a rule, a plurality of planet-wheel trains which are activated by means of clutches or brakes. As is known, with such automatic multi-step transmissions, gear changes can be carried out under load without any interruption in traction. One disadvantage of this type of automatic transmission is the lower efficiency which is caused essentially by the hydrodynamic torque converter. To improve the efficiency, the converter can in many instances be bridged by means of a lock-up clutch.
Moreover, in the field of automatic transmissions, there are what are known as continuously variable transmissions. The mechanically continuously variable wrap-around transmissions with steel band (CVT) are the furthest advanced in this context.
Finally, what are known as double-clutch transmissions have also been known for many years. These have two parallel force transmission branches and two parallel starting and separating clutches which can be activated in an overlapping manner. By means of this type of transmission, too, gear changes can be carried out under load without any interruption in traction. However, the outlay in structural terms is very considerable because of the two clutches. Furthermore, double-clutch transmissions cannot be controlled in a simple way in regulating terms.
A further type of automatic variable-speed transmission is known from DE 197 41 440 A1. The variable-speed transmission has a first torque-transmitting device which is arranged between the drive shaft and the output shaft and allows various transmission ratios and which, in one operating mode, ensures a complete force-flux uncoupling of drive shaft and output shaft. Parallel to the first device, a second device for torque transmission between the drive shaft and the output shaft is provided, by means of which, in contrast to the first device, a continuously selectable torque in the range of 0 to the maximum prevailing torque can be transmitted. However, the variable-speed transmission has only four gears and, overall, a relatively low spread. Moreover, the efficiency is open to improvement.